Structural evolution of the Mokrsko-West, Mokrsko-East and Celina gold deposits, Bohemian Massif, Czech Republic: Role of fluid overpressure
Abstrakt
The studied Mokrsko-West (90-100 t Au), Mokrsko-East (30 t Au) and Celina (11 t Au) deposits represent three spatially and genetically interrelated deposits of supposed affiliation to the intrusion-related gold deposit type. The deposits differ in their dominant host rocks, which are represented by ca 354 Ma old biotite tonalite (Mokrsko-West) and Neoproterozoic volcanic and volcanosedimentary rocks (Mokrsko-East, Celina).
Another difference lies in the style of veining densely spaced networks of 0.1-5 mm thin veins (Q(2)) within the tonalite, compared to thick (usually 5-20 cm; Q(1-2)) and widely spaced veins within the Neoproterozoic rocks. Five generations of quartz veins, referred to as Q(0) through Q(4) were distinguished: Q(0) veins are the oldest and ore barren, Q(1) veins mark the onset of the Au-ore formation, Q(2) veins its culmination and Q(3) veins its fading.
Late quartz gangue (Q(4)) is associated with uneconomic Ag-Pb-Zn vein-type ores hosted by calcite-barite-(quartz) veins. Quartz vein thickness (similar to 0.3 to similar to 300 mm), spacing (similar to 3 mm to similar to 500 mm), distribution, and related extensional strain (ca. 3-25%) evolve systematically across the studied ore district, reflecting both the major host rock and other tectonic factors.
Detailed study of vein dimension parameters (thickness, length, width, aspect ratios) allowed estimation of the probable depth of the fluid source reservoir (similar to 2 km or similar to 4 km) below the present surface. The depth to the fluid source seems to increase through time, being the shallowest for the Q(0) veins and the deepest for the Q(2) veins.
Two independent methods of estimating fluid overpressure are discussed in the paper. Fluid overpressure during vein formation decreases from the Q(0) through the Q(2) veins, from 10 to 4 MPa or from 26 to 10 MPa, depending on the assumed tensile strength of the tonalite (5.5 and 15 MPa, respectively).